Your hot and cold water are actually connected anywhere you have a single handled faucet, but the problem doesn’t usually manifest with MORE cold water coming out when you flush the toilet.
Anywhere you have a single handled faucet, you have one cartridge, if it breaks, you’ve effectively connected your entire system. Also, in your shower, you have an anti-scald valve which has a rubber diaphragm that can break. But like I said, above, more cold water coming out when there’s a pressure drop on the cold water side isn’t the typical symptom of any of these things going bad.
The typical complaint from a home owner is that cold water from a specific faucet (the one nearest the problem) gets them tepid/warm water.
The toilet flush lowers the pressure in the hot water lines enough that the cold water dominates. I’m assuming it’s a single handle mixer on the shower/tub. The extra height of the shower allows that to happen while it doesn’t affect the tub (though I’d bet the output from the tub does get colder). You might be missing a back flow preventer somewhere, or the pressure bladder on your hot water tank isn’t working. You might think the pressure loss would be in the cold water lines, but they’re generally less complex and take a shorter route than the hot water lines. A plumbing expert could tell you about the pipe configurations that make that likely to happen.
It’s just a “water closet”. A tiny, tiny little room (closet, actually) in the garage that has a functioning urinal in it. No sink/toilet/shower or anything else. Don’t know if it was originally here when the house/garage were built, but it was there when we bought it. It’s to the side parallel to how cars are parked, with a door, so not something you can see when the garage door is opened or anything like that.
The garage, while 2 car, is actually small. All my tools/snow blower/lawn mower, etc. are kept in a yard barn. There really is nothing to do in the garage other than get in/out of a car so taking a leak there is an extremely rare occurrence.
Are you sure?
I have a water tempering valve on my toilet and it has malfunctioned in the past.
That and the new hot water heaters have a flow control device/valve in the output connection. I removed this in the new heater I put in last winter.
And a blast of cold water is so much better than a blast of scalding hot water.
My opinion on this problem may require a couple of flow controls and that could be check valves or gate valves set at the best flow.
